There's a French term for this, deja vu. We don't know exactly why it happens, but many people experience this feeling.

There is one theory that our brains have two settings for events, "this is happening right now" and "this already happened" and that normally, our memories of events go from one category to the other after they conclude. After we're finished with something, our brain stores it in the past category. The idea is that deja vu might be what happens when our brain gets that process mixed up, and we feel that the event is happening right now, and already happened at the same time.

remember you don't have any access to objective reality. everything you see is your brain's reconstruction of nerve signals coming from light hitting your retinas. optical illusions happen because sometimes your brain misinterprets the inputs, which highlights the fact that what you're seeing isn't "real" but a representation of reality that isn't always accurate.

your memories are stored in your brain, they're also not real and they don't necessarily correspond to reality. you can make time slow down through certain methods that cause data to load into your short term memory incorrectly. time isn't really slowing down, but your brain's stretching out the experience in your memory and since you live in brain-world, not real-world, you experience the stretched reality.

so if your brain accidentally sticks something in your short term memory before it consciously registers the thing, once it registers the thing it looks back and says "oh that already happened." it has no way of accessing the real world to correct itself, it can only trust its own reconstructions.

I read an article about a study int deja vu. It basically said it was a memory short circuit. Every day you brain loads everything that happened into short term memory. At some point during the day, usually while you sleep, your brain processes all these memories and decides what needs to go to long term or permanent memory, everything else is tossed to the side. So, whenever you do something your brain scans your long term memory to see if you have done this before. If you have it can provide useful details. If not it loads it into short term memory and the process starts all over.

The important thing is whenever you do something your brain scans long term memory to see if you have done this before. Deja vu can happen if your brain hiccups and for some reason starts loading whatever you are doing at the time into long term memory. So your brain is scanning long term memory as you are writing to long term memory. It makes what you are doing at that exact moment feel like a memory even though it isn’t. It’s also why you get that weird feeling because your brain knows something is not right and it’s trying to figure out what it is.

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In addition to what other people have posted, you could also get deja vu simply because you have in fact done something like what you are currently doing before, even if you don't remember precisely when. I mean, if you walk down a street and see a church, and get a feeling of deja vu, well, you almost certainly have walked down streets and seen churches before. The brain makes the connection but fails to pull up a specific example, so you get that feeling.

Not to worry you, but if it is happening quite often, please ask others around you to make sure you seem ok and responsive when you experience it because deja vu is actually very common in people with seizures/epilepsy (particularly epilepsy that comes from misfiring in the temporal lobe). Would be worth mentioning to a doctor if this is happening frequently.